Reuse lexer and parser tests from beancount

[mbudde]

Reuse the test examples from lexer_text.py and grammar_test.py in the beancount source code to test that our parser accepts and rejects the same inputs as the official beancount parser.

[vHanda]

Warning: Beancount is GPL licensed. Reusing the tests might mean that this project cannot be MIT / Apache2 licensed.